1. Field of the Invention
The present invention relates to a motor for a heat dissipating fan and, more particularly, to an inner rotor type motor for a heat dissipating fan. The present invention also relates to a heating dissipating fan including such an inner rotor type motor.
2. Description of the Related Art
FIG. 1 shows a conventional heat dissipating fan 7 including an outer rotor type motor. The heat dissipating fan 7 includes a housing 71, a stator 72, a circuit board 73, and a rotor 74. A shaft tube 711 is formed in the housing 71 and receives at least one bearing 712. The stator 72 and the circuit board 73 are mounted around the shaft tube 711 with the circuit board 73 electrically connected to the stator 72. The rotor 74 includes a shaft 741 and an impeller 742. An end of the shaft 741 is extended into the shaft tube 711 and rotatably held by the bearings 712. The impeller 742 is coupled to the other end of the shaft 741 and includes a permanent magnet 743 facing the stator 72. The circuit board 73 controls the stator 72 to generate an alternating flux, and, with the interaction of the alternating flux, the permanent magnet 743 will be driven to rotate. Thereby, the impeller 742 rotates for heat dissipation purposes.
Motors of the outer rotor type are generally less stable and have lower speed in rotation than motors of the inner rotor type. The current trend of research and development of electronic products includes high speed, function integrity, and miniaturization, which also have an increased need in heat dissipation. Unfortunately, the conventional heat dissipating fan 7 driven by an outer rotor type motor can not fulfill the heat dissipation need of these electronic products.
To meet higher heat dissipating needs, heat dissipating fans having inner rotor type rotors are utilized, and an example of which is disclosed in Taiwan Patent Publication No. 200744290 entitled “FAN AND ITS INNER ROTOR TYPE MOTOR.” As illustrated in FIG. 2, the heat dissipating fan 8 includes a housing 81, a stator 82, a rotor 83, a driving device 84, and an impeller 85. The housing 81 is comprised of first and second housing parts 811 and 812 each receiving a bearing 813. The stator 82 includes a magnetically conductive member 821 to which the driving device 84 is electrically connected. The rotor 83 includes a shaft 831 and a magnetic element 832. The shaft 831 extends through the housing 81 and is rotatably held by the bearings 813. The magnetic element 832 is mounted around the shaft 831 and faces the magnetically conductive member 821. The impeller 85 couples to the shaft 831 and is arranged outside the housing 81. However, the shaft 831 of the heat dissipating fan 8 must include a portion L1 for mounting the magnetic element 832 and portions L2 and L3 for coupling with the first and second housing parts 811 and 812 and the bearings 813 for the purpose of stable rotation. Furthermore, the portion L1 must exceed a certain length to assure a sufficient area for magnetic interaction between the magnetic element 832 and the magnetically conductive member 821. Thus, it is difficult to reduce the length of the shaft 831, which limits reduction of the overall axial height of the inner rotor type motor and the heat dissipating fan 8 including the inner rotor type motor. Compact designs are, thus, not allowed.
FIG. 3 shows another conventional heat dissipating fan 9 driven by an inner rotor type motor 92. The heat dissipating fan 9 includes a housing 91, the inner rotor type motor 92, an impeller 93, and a circuit board 94. The housing 91 is integrally formed from plastic material by injection molding and includes a fan housing portion 911 and a motor casing 912. The motor casing 912 is located in the fan housing portion 911 and receives two bearings 913. The inner rotor type motor 92 is mounted in the motor casing 912 and includes a shaft 921, a magnetic element 922 and a stator 923. An end of the shaft 921 extends beyond the motor casing 912. The magnetic element 922 is mounted around the shaft 921 and faces the stator 923. The impeller 93 is coupled to the end of the shaft 921 and located in the motor casing 912. The circuit board 94 is electrically connected to the stator 923. The inner rotor type motor 92 drives the impeller 93 to rotate for heat dissipation purposes.
The heat dissipating fan 9 is advantageous over the heat dissipating fan 8 by the housing 91 that guides the air currents created by rotating the impeller 93. However, the shaft 921 of the heat dissipating fan 9 still has to include a portion L1 for mounting the magnetic element 922 and portions L2 and L3 for coupling with the motor casing 912 and the bearings 913 for the purpose of stable rotation. Likewise, it is difficult to reduce the length of the shaft 921, which limits reduction of the overall axial height of the inner rotor type motor 92 and the heat dissipating fan 9 including the inner rotor type motor 92.